gdb: handle calls to edit command passing only a linespec condition

While working on the previous commit to fix PR cli/28665, I noticed
that the 'edit' command would suffer from the same problem.  That is,
something like:

  (gdb) edit task 123

would cause GDB to break.  For a full explanation of what's going on
here, see the commit message for the previous commit.

As with the previous commit, this issue can be prevented by detecting,
and throwing, a junk at the end of the line error earlier, before
calling decode_line_1.

So, that's what this commit does.  I've also added some tests for this
issue.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28665